Prism-coupling techniques can be used to measure the spectrum of guided modes of a planar optical waveguide to characterize the waveguide properties, e.g., to measure the refractive index and stress profiles. This technique has been used to measure the stress and the depth-of-layer (DOL) of different ion-exchanged (IOX) glasses used for a variety of applications, such as for cover glasses for displays (e.g., for smart phones).
Certain types of IOX glasses are actual dual IOX (DIOX) glasses formed by first and second diffusions that give rise to a two-segment profile. The first segment is adjacent the surface and has a relatively steep slope, while the second segment extends deeper into the substrate but has a relatively shallow slope. Such DIOX profiles are used for certain types of chemically strengthened glasses and anti-microbial glasses.
Such two-segment profiles result in a relatively large spacing between low-order modes, which have a relatively high effective index, and a very small spacing between high-order modes, which have a relatively low effective index. The spacing between modes is of interest since the modes are detected as lines in a mode spectrum by a photodetector (i.e., a digital camera). The resolution of the measurement is defined by the number of photodetector pixels between adjacent modes.
This distribution of the modes over the photodetector pixels is problematic when precise measurements of the deeper segment of the DIOX profile are sought because the high-order modes that travel in the deeper segment are under-sampled. If the DOL is large enough (e.g., greater than 70 μm or 100 μm or even greater than 140 μm or 150 μm,) it becomes impossible to adequately resolve the spectral lines of high-order modes, and as a result the DIOX profile cannot be precisely measured. In addition, for large DOL, the waveguide shape starts to become an issue and can adversely affect measurement quality.
One option for obtaining the required measurement resolution is to have a larger photodetector with more pixels, which in some cases may also require a larger-aperture optical system. However, such photodetectors and larger-aperture optical systems add substantial cost and complexity to the measurement system.